Soybean is the leading oilseed crop produced and consumed worldwide (Wilcox World distribution and trade of soybean, 2004, Soybeans: Improvement, production, and Uses, 3rd ed., Agron. Monogr. 16, ASA, CSSA, and SSSA, Madison, Wis. p: 1-14; Hymowitz, 2004, Speciation and Cytogenetics, p. 97-136. In H. R. Boerma and J. E. Specht (ed) Soybeans: Improvement, production, and Uses, 3rd ed. Agron. Monogr. 16. ASA, CSSA, and SSSA, herein incorporated by reference in its entirety). In the past half century, the USA was the world's leading producer. In 2003, the USA produced 35% (65.8 million metric tons (MT)) of the world's total soybean (FAOSTAT, 2004, Production Crops). However, soybean plants have many insect pests, including the soybean aphid, limiting its production in other parts of the world around 2000 A.D., the soybean aphid (Aphis glycines Matsumura) a native to eastern Asia, became a major sucking pest of soybean [Glycine max (L.) Merr.] in North America. Since then, this insect pest has rapidly spread to the major soybean production areas in the USA and Canada (Plant Health Initiative, 2004, Soybean Aphids Research Update “Aphids discovered in Wisconsin” from the North Central Soybean Research Program (NCSRP) published online by the Plant Health Initiative Available at planthealth.info/soyaphid.htm (verified Oct. 5, 2004), herein incorporated by reference. Aphid outbreaks were severe in the northern part of the Midwestern USA and in Ontario, for example in years 2001 and 2003.
Several factors affect soybean aphid outbreaks, including environmental conditions, over-wintering success, cultural practices, natural enemies, and the synchronization of soybean and aphid development (Wu et al., 1999, How Peasants Can Increase Wealth [Nongmin zhifu zhiyou] 6:20, herein incorporated by reference). The soybean aphid develops large colonies on soybean plants. Plant damage occurs when large numbers of aphids remove significant amounts of water and nutrients as they feed (suck) on leaves and stems, causing leaves to wilt, curl, yellow, and even drop off. Other symptoms of direct feeding damage include plant stunting, poor pod fill, reduced pod and seed counts, smaller seed size, and nutrient deficiencies resulting in overall yield and quality reduction (DiFonzo and Hines, 2002, Michigan State University Extension Bulletin E-2746, herein incorporated by reference). Significant yield loss (8-25%) occurred when the aphid heavily infests the soybean plants during the early reproductive stage (DiFonzo and Hines, 2002, Michigan State University Extension Bulletin E-2746, herein incorporated by reference). Honeydew, a sticky substance excreted by soybean aphids onto the leaves, leads to the development of sooty mold, which affects photosynthesis and results in yield loss (Baute, 2004, (Soybean Aphid Factsheet and Soybean Webpage sponsored by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), published online). During the feeding process, soybean aphids are capable of transmitting viruses into host plants including alfalfa mosaic virus, soybean mosaic virus, and bean yellow mosaic virus. These viruses commonly occur together and form a disease complex that leads to plant stunting, leaf distortion, leaf and stem mottling, reduced pod numbers, and seed discoloration (Glogoza, 2002, North Dakota State University Extension Bulletin E-1232, herein incorporated by reference). Soybean aphids also feed on other crop plants such as beans and pumpkins while spreading viral diseases such as cucumber mosaic virus, zucchini yellow and watermelon mosaic between vegetable crops.
Aphids are particularly difficult to control because of their rapid reproduction rates and ability to disperse over wide areas. Populations build rapidly, for example, females give live birth to already pregnant young aphids that mature in 3-7 days, with a doubling time of 2-5 days under favorable conditions. Winged forms appear and disperse to other fields under high insect densities and when infested plants are stressed. Since aphids are relatively weak fliers, long-distance dispersal is often at the mercy of prevailing winds.
Aerial applicators of insecticides frequently report having to stop to clean their windshields from flying into clouds of these aphids above heavily infested fields. In 2001, the influx of winged soybean aphids into the open dome of the Toronto Blue Jays even caused an early end to a Toronto Blue Jays game. It's these flights that lead to rapid, progressive colonization of soybean fields, almost like a wave moving across the countryside. Under favorable conditions for aphid infestations, the settling of winged aphids into uninfested fields has been described as “aphid rain.” (Ostlie, Soybean Aphid Pages published online by Just for Growers, MN (University of Minnesota) Soybean Production, published online by the University of Minnesota, the University of Minnesota Extension Service, and the MN Soybean Research and Promotion Council Jul. 6, 2004, herein incorporated by reference).
Insecticides are the primary available method of controlling soybean aphids in the United States. Although the use of insecticides can be a quick and easy way to control aphids, the ideal time to spray is not well defined and the insecticides may not have long term action. When applications of insecticides are poorly timed or applied incorrectly then aphid populations may resurge and further may develop resistance to the insecticide. The use of insecticides also has many adverse effects such as killing beneficial insects, environmental contamination, and increased production costs (Sun et al., 1991, Soybean Sci. 10(2): 98-103, herein incorporated by reference).
Another way of attempting to control soybean aphid infestations is by importing and releasing a natural enemy of the soybean aphid, such as Binodozys communis, a tiny stingless wasp. However these wasps may not over winter in large enough numbers to provide significant control in colder climates, such as the northern United States and Canada. Additionally, when a large number of flying aphids infest fields they would overcome smaller numbers of predator wasps. Testing is underway to determine whether these alien wasps will also attack beneficial insects.
Therefore there is a need for developing aphid resistant soybean plants having a higher resistance to aphids as a long-term solution to the aphid infestation of commercially grown soybean plants.